1. Field of the Invention
This invention relates to a diaphragm for a speaker and, more particularly, to an improvement in a material constituting such a diaphragm.
2. Description of the Prior Art
Greater lightness, larger hardness or stiffness, and larger internal loss (periodic damping) are requirements in the characteristics of a diaphragm for a speaker. More specifically, the uniform operation of a diaphragm is desired in a frequency range as wide as possible with respect to an input signal in order to reproduce a high fidelity sound by the speaker. Accordingly, it is advantageous for the diaphragm to be lighter and harder. In other words, it is advantageous that the ratio E/.rho. be larger where E is Young's modulous and .rho. is density. Also, the internal loss must be larger to prevent undesirable resonance.
A conventional diaphragm for a speaker could not satisfy all the desired features sufficiently. For example, although hard paper which was widely in common use had an advantage of lightness, it was inferior in hardness. Also, a light metal such as aluminum, titanium, beryllium, and the like was used as a diaphragm of a tweeter speaker of a small diameter. However, it was required to retain the bending strength across a large area of the diaphragm in order to use the diaphragm as a woofer speaker of a large diameter. Accordingly, the thickness of the diaphragm was required to be increased, thus resulting in an increased mass of the diaphragm as a whole. The increased mass thereof became an obstacle in the application of the light metal such as aluminum, titanium, and the like to a speaker of a large diameter. As a method of applying these light metals to a diaphragm of a speaker of a large diameter, a construction is proposed by Barlow, "The Development of a Sandwich Construction Loudspeaker System," Journal of the Audio Engineering Society, June 1970, Vol. 18, No. 3, wherein a porous synthetic resin is used as a dampling material (core material) and it is sandwiched between light metals such as aluminum, and the like. However, such a method does not show the nature of the metals completely.
Also, in the conventional paper-made diaphragm and metal-made diaphragm, the physical conditions of the material restrict the design conditions of the speaker unit. Thus, changes in the physical conditions of the diaphragm restrict the free setting of the acoustic characteristics.
A prior art of interest to this invention may be found in Japanese Utility Model Laying-Open No. 61025/1975 dated June 5, 1975. In brief, it discloses an electro-acoustic transducer diaphragm comprising a metal base material having continuous pores filled with an organic compound which provides a high internal loss, the surface of said base material being formed with a layer of metal plating. As for the metal base material having continuous pores, porous metals, such as aluminum and iron oxides, having an apparent specific gravity of about 0.3-0.5 are taught therein. The cited gazette makes no mention of a method of producing such base materials. In the above referenced gazette, the organic compound is incorporated for the purpose of increasing the internal loss of the diaphragm and latexes such as acrylonitrile butatadiene copolymer, styrene butadiene rubber and ethylene vinyl chloride are given as examples thereof. The composite comprising a metal base material having continuous pores filled with an organic compound which provides a high internal loss has a layer of metal plating formed thereon for the purpose of adding to the mechanical strength of the base material. The layer of metal plating may contain copper, nickel, chromium or various alloys. The method of forming such layer of metal plating disclosed therein comprises the first step of activating the surfaces of the base material and organic compound, the second step of performing non-electrolytic plating and the third step of performing electroplating.
The porous metal used in the electro-acoustic transducer disclosed in the above referenced gazette is disadvantageous in that it has a high Q value. Generally, Q value is related to the internal loss, and materials with high Q values have low internal losses. Generally, metals have high Q values and hence low internal losses since the resistance component across the grain boundary is small. In order to improve the characteristics of a speaker diaphragm, higher internal loss is preferable. The electro-acoustic transducer disclosed in the above referenced gazette uses as a base material a metal whose porosity is low. As a result, the feature of metals having high Q values is passed on to the transducer diaphragm. Therefore, with a porous metal alone, it is impossible to provide a speaker diaphragm having desired characteristics. It is because the inventor in the above referenced gazette was aware of this disadvantage that he filled the pores with an organic material to increase the internal loss.
The electro-acoustic transducer disclosed in the above referenced gazette has the pores filled with an organic material of high internal loss for the purpose of improving the characteristics dependent on internal loss, as described above. However, organic materials of high internal loss generally have high specific gravities, so that the filling with said organic material obviates the feature of porous metals being light weight. The electro-acoustic transducer in the above referenced gazette is designed to have an increased mechanical strength by plating with a heavy metal such as copper, nickel, or chromium the composite comprising a porous metal such as aluminum and an organic compound of high internal loss filled into the pores therein. When aluminum and nickel foils having the same weight are considered, the aluminum foil has about ten times the rigidity, or stiffness, of the nickel foil. Further, when the rigidities provided by a foil and by a layer of plating are compared, the foil, of course, provides a higher rigidity. On the other hand, when aluminum and nickel foils having the same degree of rigidity are compared, the nickel foil is heavier. With all these taken into consideration, it may be said that although the transducer disclosed in the above referenced gazette may have an increase in mechanical strength provided by nickel plating, it has a disadvantage that the resulting increase in weight kills the feature of the porous metal being light weight. Thus, plating a composite including a porous metal with a heavy metal cannot avoid increasing the weight. Furthermore, plating requires that the surface of an object to be plated be treated in advance to provide for plating. Therefore, the disclosed transducer needs such processes as surface activation and non-electrolytic plating, resulting in an increase in the number of processing steps and increased cost. Further, the disclosed transducer may undergo a large volumetric change due to heat and the organic material used as the filler, so that it is liable to suffer the deformation of the speaker diaphragm and the variation of the characteristics.